The Caledonian–Appalachian Orogen was formed by the closing of a Paleozoic Iapetus Ocean. The continental margins of Iapetus are identified in the deformed early Paleozoic miogeoclines of the Caledonian–Appalachian Orogen. Ophiolitic vestiges of Iapetus, its oceanic plateaus, microcontinents, and volcanic arcs are Caledonian–Appalachian suspect terranes. These were assembled in interior parts of the orogen and locally they were emplaced structurally upon the adjacent miogeoclines.The modern North Atlantic Ocean opened along an axis that traversed the Paleozoic orogen longitudinally. Its opening dispersed the elements of the Paleozoic orogen and led to the present arrangement of disjunct Paleozoic miogeoclines and suspect terranes throughout the North Atlantic borderlands.The western or North American margin of Iapetus is represented by the miogeoclines along the west flank of the North American Appalachians and Caledonides of east Greenland. A small North American miogeoclinal segment occurs in the British Isles, and suspect terranes with North American faunal affinities occur in Scandinavia. The eastern margin of Iapetus is represented by the miogeoclines of the Scandinavian Caledonides and the Mauritanides of northwest Africa. Ophiolitic vestiges of Iapetus and suspect terranes occur in the Appalachians, the Caledonides of Scandinavia and the British Isles, and the Variscan foldbelt of Morocco, Iberian Peninsula, and western France.In the scenario of a closing Iapetus and opening North Atlantic, the Paleozoic margin of eastern North America expanded by the acquisition of Appalachian suspect terranes, the Paleozoic margins of Greenland and Scandinavia remained essentially unchanged, and Africa lost parts of its Paleozoic margin.Modern continental margins and the geometry of the North Atlantic mimic Paleozoic miogeoclines and the geometry of Iapetus. The Paleozoic miogeoclines, in turn, follow Grenvillian deformed zones of the Precambrian North Atlantic craton. Thus, patterns for the opening of the North Atlantic may have been set by the geometry of the Grenvillian deformed zones.